There has heretofore been a printing head in which a plurality of printing wires are electromagnetically driven to form a dot-matrix record on a recording medium which is on a platen, through an ink ribbon with the front ends of the printing wires. Recently, with the demand for a higher printing density, the number of printing wires has increased. For example, there is a printing head having 24 printing wires which are aligned in two rows in a direction substantially perpendicular to the direction of movement of the printing head. The front end portion of each printing wire extends through a predetermined guide hole in a front guide.
There are two different arrangements of printing wires. One is a parallel arrangement in which two wires which are adjacent to each other in the direction of movement of the printing head are at the same level. The other is a staggered arrangement in which two wires which are adjacent to each other in the direction of movement of the printing head are offset from each other by a half-pitch. The parallel arrangement is suitable for high-speed printing (draft mode), while the staggered arrangement is suitable for high-density printing (NLQ (Near Letter Quality) mode).
In the prior art, some arrangements which enable the parallel and staggered arrangements to be changed over from one to the other as desired have been disclosed.
Firstly, U.S. Pat. No. 4,010,835 (Japanese Patent Application Post-Examination Publication No. 57-60950) discloses an arrangement as a specific means for attaining the above-described purpose, in which a wire guide that supports one row of printing wires is arranged to be fixed, while another wire guide that supports another row of printing wires is arranged to be movable vertically by a solenoid, and the two guides are disposed adjacent to each other. When the movable wire guide is moved upwardly, a staggered arrangement is formed, whereas, when it is moved downwardly, a parallel arrangement is formed. As another specific means, a single wire guide is arranged to support all printing wires and to be rotatable by a small amount either clockwise or counterclockwise. When the printing wires are aligned in left-downwardly inclined form by clockwise rotation, a parallel or staggered arrangement is obtained. When the printing wires are aligned in right-downwardly inclined form by counterclockwise rotation, the other arrangement is obtained.
Secondly, U.S. Pat. No. 4,470,713 (Japanese Patent Application Post-Examination Publication No. 2-29022) discloses an arrangement in which a front guide is formed of three plates which are adjacent to each other, and two adjacent plates of the three are used as fixed guide plates, while the other plate is used as a movable guide plate. Guide holes are provided as follows: In the fixed guide plates, semicircular recesses are formed in the opposing end faces in opposing relation to each other, and one row of printing wires on the fixed side are supported by the recesses. Another row of printing wires are supported by recesses provided in the opposing end faces of the fixed and movable guide plates.
More specifically, semicircular recesses are formed in the end face of the movable guide plate, and the printing wires of the second row are disposed in the recesses, while the end face of the fixed guide plate has small irregularities on the respective bottoms of the recesses so that the printing wires of the second row are allowed to shift by a half-pitch. Further, in order to limit the stroke of the movable guide plate, semicircular recesses are provided in the movable guide plate at the upper and lower sides of the row of printing wires, and shafts each having the same diameter as those of the printing wires are secured in the recesses. The fixed guide plate is formed with recesses at positions which face the shafts, respectively. Each of the recesses comprises two circular arcs having the same curvature radius as that of the holes receiving the printing wires. The center distance of the two circular arcs is half the center distance of each pair of adjacent holes receiving printing wires. Each shaft selectively fits into either of the two circular arcs. Thus, the shaft can move vertically by a half-pitch. The movable guide plate is secured to a support member, which is biased toward the fixed guide plates by a spring force. Further, the support member is provided with a finger-shaped member. Left and right side plates which the printing head faces when moved leftwardly and rightwardly, respectively, are provided with upwardly and downwardly inclined surfaces, respectively. Accordingly, when the finger-shaped member engages with the upwardly inclined surface, the movable guide plate moves upwardly, so that the printing wires are arranged in the staggered pattern, whereas, when the finger-shaped member engages with the downwardly inclined surface, the movable guide plate moves downwardly, so that the printing wires are arranged in the parallel pattern.
Thirdly, U.S. Pat. No. 4,605,323 discloses an arrangement in which two rows of printing wires are supported by respective guides which are movable vertically independently of each other. The two guides are mounted on a plate which is moved forwardly and backwardly by a solenoid. The upper side of this plate is formed with two inclined surfaces where the two guides are mounted, respectively. The two inclined surfaces are opposite to each other in the direction of inclination. Accordingly, when the plate moves forwardly, one guide moves upwardly, while the other guide moves downwardly, whereas, when the plate moves backwardly, the first guide moves downwardly, while the second guide moves upwardly. Thus, the relative positions of the two rows of printing wires are changed over from those for a staggered arrangement to those for a parallel arrangement, or vice versa.
Fourthly, German Patent Application Laid-Open No. 3,208,104 discloses an arrangement in which a rotatable front guide is rotated a little either clockwise or counterclockwise, thereby changing over the arrangement of printing wires to a parallel arrangement in which the printing wires align in a left-downwardly inclined form or a staggered arrangement in which the printing wires align vertically. A mechanism that is actually used to rotate the front guide is described below.
That is, a sliding bar is connected to the front guide, and a solenoid is disposed at the outside of a base plate of the printer. The arrangement is such that when the carrier stops in close proximity to the base plate, the sliding bar projects from the base plate. Then, the solenoid is driven to project the plunger so as to engage with the sliding bar. At this time, when the printing head is moved a little toward or away from the base plate, the front guide rotates a little clockwise or counterclockwise through the sliding bar, thereby allowing the parallel and staggered arrangements to be changed over from one to the other.
Fifthly, Japanese Patent Application Laid-Open (KOKAI) No. 63-99960 discloses an arrangement in which guide hole arrangements of two front guides are partly or wholly displaced from each other to thereby incline the corresponding wires. In addition, the gap between the head and the platen is changed to thereby change the position where dots are formed by the front ends of the wires to a position corresponding to the parallel arrangement or the staggered arrangement, thereby allowing the high-speed and high-density printing modes to be changed over from one to the other.
The above-described first prior art requires a solenoid for vertically driving one wire guide and therefore provides an increase in the cost. In the arrangement where the front guide is rotated, since the arrangement of the printing wires is not perpendicular to the direction of movement of the printing head, whether in the parallel arrangement or the staggered arrangement, the processing of printing data for driving the wires becomes complicated, so that the required memory capacity increases, and the processing speed must be increased. Thus, the cost of the control circuit rises. In addition, printing wires which are close to the center of rotation of the front guide and those which are remote from it have different amounts of torsion applied to them caused by the rotation of the front guide, and this causes variations in the wire characteristics.
In the second prior art, the printing wires in one row are supported by the opposing semicircular recesses, while the printing wires in the other row are supported by the semicircular recesses and the small irregularities. Since these printing wire support portions are not perfect holes, the printing quality degrades. In addition, small irregularities are readily worn during use. If the small irregularities are worn, the support of this row of printing wires becomes insufficient, resulting in a lowering in the printing quality. Further, it is likely that ribbon tailings, ink, printing paper dust, etc. will adhere to the recesses of the fixed guide plates. Therefore, there is a likelihood that the shafts of the movable guide will become unable to move satisfactorily due to adhesion of such dust, resulting in a failure to change over the printing modes.
The third prior art requires a solenoid for vertically moving the two guides and therefore provides an increase in the cost.
In the fourth prior art, since the front guide is rotated, printing wires which are close to the center of rotation and those which are remote from it have different amounts of torsion applied to them, which causes variations in the wire response characteristics. Further, since a solenoid is needed, the cost increases.
In addition, since the pitch of the printing wires in the vertical direction in the staggered arrangement differs from that in the parallel arrangement, the character size varies undesirably.
In the fifth prior art, since the printing wires are inclined by two front guides, the load applied to the wires increases, giving rise to problems in terms of response characteristics and durability. In addition, since the wire stroke in the parallel arrangement differs from that in the staggered arrangement, a difference is likely to be produced in printing characteristics.